5cm, put it facing down, total was like, 7.6cm, 5cm sticking above the washeri stuck metal bar across the top, so pusshing downI put bucket on one side and i pushed down on the other side. It broke at 3kg, so since I balanced the torque, my force also was 3kg. 6kg. so...at 6kg at 5cm, the torque was great enough to break it..idk what type of break it exactly is...it bent, because the balsa core inside sheared...if that makes sense..near the bottom..I'm making it under 1 cm..since .7cm, about 7 times less distance, i was hoping it'd withstand 7times more, so 6kg*7=42kg..about what it would experience.

If I am afraid i am not building perfectly..and then the compression wouldnt be exacly 90 with the wall, looking at the X plane, then taper would be benficial then? Since that would kind of be like, off-set force, not due to swaying bucket or uneven table, but my inprecise building?

I don't know if this has been brought up already, but if a balsa stick is very strong in compression, would it theoretically be strong in tension too? Currently, I select my competition pieces with three criteria in mind: mass, buckling strength, and straightness. However, I have several pieces on my boomilever that are pure tension pieces, so I'm not sure if I should be selecting them due to their buckling strength.

Thanks in advance.

University of Pennsylvania Class of 2020Strath Haven High School Class of 2016

havenguy wrote:I don't know if this has been brought up already, but if a balsa stick is very strong in compression, would it theoretically be strong in tension too? Currently, I select my competition pieces with three criteria in mind: mass, buckling strength, and straightness. However, I have several pieces on my boomilever that are pure tension pieces, so I'm not sure if I should be selecting them due to their buckling strength.

Thanks in advance.

Read up on grain of the wood. Aia mentions it and specializedbalsa has a page on it. Basically, C grain for compression and A grain for tension. But you should research why to understand the design better. Also, there are other issues with tension such as shearing.

havenguy wrote:I don't know if this has been brought up already, but if a balsa stick is very strong in compression, would it theoretically be strong in tension too? Currently, I select my competition pieces with three criteria in mind: mass, buckling strength, and straightness. However, I have several pieces on my boomilever that are pure tension pieces, so I'm not sure if I should be selecting them due to their buckling strength.

Thanks in advance.

Read up on grain of the wood. Aia mentions it and specializedbalsa has a page on it. Basically, C grain for compression and A grain for tension. But you should research why to understand the design better. Also, there are other issues with tension such as shearing.

I have read up on it in the past, and considering the only balsa grain I have is A grain (some B grain mixed in, though), I don't have many options to pick out my grain.

Is a higher buckling strength related to a higher tension strength, though?

University of Pennsylvania Class of 2020Strath Haven High School Class of 2016

I don't think that's quite what he's asking. It's an interesting question though, the buckling strength does tell you things about the wood(mainly modulus of elasticity) that density doesn't tell you exactly... And I'm not an engineer or anything so I'm probably wrong... but... The elastic modulus used in Eulers Buckling theorem is Young's Modulus(?) which is the ratio of the stress put on a piece to the strain in tension(basically how far it would stretch in tension), so there would be a relation between compressive strength and tensile strength. But again... I'm not entirely sure

Either way I've been fine all year just using mass for my tension pieces (and obviously grain orientation)

'If you're the smartest person in the room, you're in the wrong room' - Unknown

At a given cross section, both tensile strength and buckling strength are a function of density. Increase density, both go up.

Looking at tensile strength first. There’s very little published data I’ve been able to find out there on density vs tensile strength. The data I’ve seen suggest the relationship is linear, or close to linear. Double density, and the same cross section, tensile goes up by a factor of just a bit over 2; ~2.1. If you double cross section (at the same density), you double tensile strength. Some data suggest the gain in high density balsa is a bit better than linear. Length doesn’t matter- at same cross section and density, pieces at different length will have the same tensile strength. The smaller you go in cross section, the greater the risk of …..grain weakness- grain planes running out of the piece. Tensile strength for balsa compared to bass, at about the same density is somewhere between ½ and 1/3 that of balsa. Balsa pieces in tension can work fine/efficiently, but as main tension member(s), not a good choice, not only for tensile strength vs weight, but because of brittleness. Also, because of shear strength- different than tensile; it comes into play on the glue joints- what kind of joint/glue area you need to avoid a thin layer of wood just shearing away. I previously posted a length to a good, comprehensive set of data on (a wide range of) wood properties

Looking at compression and buckling strength- and it’s buckling strength that matters for pieces in compressive load.Going back to Euler, buckling strength, first, has an inverse square relationship to length – at the same density and cross section, ½ the length = 4x (2 squared) the buckling strength.At a given length, it is “E” times “I”

“E”, the modulus of elasticity/Young’s modulus, is a measure of inherent stiffness. E is a (linear) function of density; higher density, higher E. I posted a link earlier to an old US Forest Service study- has a table and graph of E vs D for balsa, over a wide range of densities. The bottom line, double density (up a factor of 2), and E goes up by a factor of about 2.8.

“I” is the cross sectional/second moment of inertia. As has been discussed at some length, for solid pieces, this just comes down to cross section dimension; double the cross section dimension (at the same density) and I doubles. For a square cross section, I is the same in both vertical and horizontal planes- a square cross section piece will buckle toward one of the four faces (up or down, or left or right), not toward a diagonal, because the face to face distances are the shortest. With a rectangular cross section- say 1/8 wide by ¼ high, I in the vertical plane is twice what it is in the horizontal plane. When you go from solid cross section to hollow cross section (box beam/tube), you can get to higher “I”s (at the same weight)

sjwon3789 wrote:My state is over and this year is over for me. For this type of event, will there be any rule changes? (I'll be in division C. Would it most likely stay the same; 15cm as height?)

Yes, we typically change the parameters of the construction events from year to year. Not sure yet what we'll do with Boomilever though.

Are there any known possibilities? It seems like making it less than 15 cm would make it impossible to hold full 15 kg, as well as length manipulation... Is something like what happened with towers, where a scoring system encompassed dimensions likely?

elispa wrote:Sorry from deviating from the original topic but what's the best place to get wood from online?

In terms of quality the best place to buy wood would be Specialized Balsa. (http://www.specializedbalsa.com/) In terms of price it's most effective to buy in sheets and use a balsa stripper to cut the size you need. If you're looking for something cheaper, places like pitsco and kelvin and balsawoodinc have less careful selection and generally do not allow you to choose densities, but will be significantly cheaper. Buying in sheets is by far the best in terms of cost and consistency as sheets are more self consistent than strips.